The field of the present invention relates to optical systems for data reading and particularly to a scanning system employing a dithering mirror actuator.
Typically a data reading device such as a bar code scanner illuminates a bar code and senses light reflected from the code to detect the bars and spaces of the code symbols and thereby derive the encoded data. In a common system, an optical beam of light, such as a laser beam produced by a laser diode is scanned over a scan angle so as to scan the laser spot across the item being read. A variety of mechanical scanning mechanisms are known as described in for example U.S. Pat. Nos. 5,475,206 and 5,629,510 or U.S. application Ser. No. 08/934,487, each of these disclosures hereby incorporated by reference, such scanning mechanisms comprising a rotating polygon mirror, dithering or oscillating mirror, scanning light source or laser diode, rotating/oscillating prisms, holographic elements and others devices. Essentially all these scanners include a supporting structure which allows for movement of the mechanical scanning mechanism.
One method commonly employed for rapidly and repetitively scanning the illumination beam across a scanned region is mirror dithering. Dithering, i.e. rapid rotational oscillation of an illumination beam steering mirror about an axis substantially parallel to the mirror face, causes the illumination beam to move rapidly back and forth generating a scan line. When this scan line illuminates a barcode, the resulting time dependent signal due to detected light scattered and/or reflected from the bars and spaces of the barcode is decoded to extract the information encoded therein. To be used in such scanning applications, the dithering motor generating the mirror motion must be stable and typically employs some sort of feedback between the motor and the motion of the mirror. Particularly for handheld scanning applications, the dithering assembly should be light, compact, reliable, and consume minimum power while producing sufficiently large amplitude motion for scanning.
The dithering mirror assembly typically includes a plastic or metal spring attached at one end to a fixed chassis mount with a mirror mount attached to the other end. The present inventor has recognized difficulties in assembling the spring assembly: gluing the spring to the chassis mount, attaching or clamping the mirror mount onto the spring, and accurately aligning and/or balancing these elements.